CHEVROLET SILVERADO SHIFTER PROBLEM

1979 Chevrolet Silverado • 120,000 miles

3 on the tree trans. Shifter will not stay in reverse in order to start and turn off the engine.
Shifter seems to be loose and falls down from reverse postion so that I guess the start switch will not work. I have to hold the lever in reverse with my left hand around the steering wheel and pull it up enough to make contact with the switch so it will start. Does it sound like something that can be adjusted or is it something that has to be replaced? An is relativly easy to make the adjustment or repair?
Thanks Rankin Bowen